1. Field of the Invention
The invention relates to a device and a method for determining a state of charge of a battery and, more particularly, to a device and a method for determining a state of charge of a battery based on changing voltage and charging current.
2. Description of the Related Art
An art for determining a state of charge of a battery based on charging voltage and charging current is known.
For example, Japanese Patent Application Laid-Open No. HEI 4-372536 discloses an art in which a battery is charged with (1) a voltage lower than an appropriate charging voltage, (2) the appropriate voltage, and (3) a voltage higher than the appropriate voltage, and a state of charge of the battery is determined by comparing the charging currents corresponding to the charging voltages (1) to (3) with the value of current produced when the battery is normal and in a charge-completed state.
In general, the charging voltage, the charging current, and the state of charge of a battery have a predetermined relationship (charging characteristic). Such a charging characteristic is used by the above-described relevant art to determine a state of charge of a battery. However, as a result of repeated charge/discharge cycles or deterioration over time, the charge characteristic (i.e., the relationship among the charging voltage, the charging current, and the state of charge) changes. Therefore, if a predetermined charging characteristic is used as a basis for determining the state of charge, high-precision determination regarding the state of charge is difficult.